Impact of Surfaces on Photoinduced Halide Segregation in Mixed-Halide Perovskites

Photoinduced halide segregation currently limits the perovskite chemistries available for use in high-bandgap semiconductors needed for tandem solar cells. Here, we study the impact of post-deposition surface modifications on photoinduced halide segregation in methylammonium lead mixed-halide perovskites. By coating a perovskite surface with the electron-donating ligand trioctylphosphine oxide (TOPO), we are able to both reduce nonradiative recombination and dramatically slow the onset of halide segregation in CH3NH3PbI2Br films. This result, coupled with the observation that the rate of halide segregation can be tuned by varying the selective contact, provides a direct link between surface modifications and photoinduced trap formation. On the basis of these observations, we discuss possible mechanisms for photoinduced halide segregation supported by drift-diffusion simulations. This work suggests that improved understanding and control of perovskite surfaces provides a pathway toward stable and high-performance wide-bandgap perovskites for the next generation of tandem solar cells.